Echo ultrasound measurements based on doppler shift or time domain correlation of two successive A-lines can be used to measure a velocity component of a flowing fluid which is parallel to the ultrasound beam axis. These techniques are, however, relatively insensitive to transverse velocity components (that is, velocity components which are normal to the ultrasound beam axis). A cross correlation technique for measuring transverse flow velocity was described in Slurry Flow Velocity, Concentration and Particle size Measurement Using Flow Noise and Correlation Techniques, Ong and Beck, Measurement and Control, Vol. 8, 1975, pp. 453-461. However, this prior art system had very poor spatial resolution and, therefore, was unsuitable for flow imaging and for diagnostic blood flow measurement in living tissue.